1. Field of the Invention
The present invention relates to a radiationxe2x80x94and/or heat-curable adhesive having high thermal conductivity.
2. Description of Related Art
Adhesives of the generic type mentioned above are commercially available. However, they do not have a satisfactory combination of properties. For example, their thermal conductivity is less than 0.7 W/mK, which is considered to be too low. Their long-term service temperature, which is below 160 degrees Celsius, is also considered to be too low. Furthermore, the radiation-cured adhesives known according to the related art have the disadvantage that they have a wet surface after curing. Atmospheric oxygen does not produce polymerization on the adhesive surface, as opposed to the remaining adhesive mass, which means that monomers remain there and do not undergo a complete reaction, which results in surface wetness.
Silicone gels are regularly applied to electric circuitry to protect them. Due to the manufacturing process used, these gels usually contain a platinum complex as the catalyst. The gelling of the gel as it comes into contact with the adhesive can inhibit the silicon gel at the boundary layer.
The thermal conductivity of a radiationxe2x80x94and/or heat-curable adhesive depends, in particular, on the concentration of the mineral filler contained in the adhesive, with the thermal conductivity rising as the filler concentration increases. However, too high an adhesive filler concentration inhibits radiation curing of the resin. In the adhesives known from the related art, the mineral filler concentration is less than 50 wt %, which does not result in a satisfactory thermal conductivity.
The object of the present invention is therefore to provide a radiation-cured and/or thermally cured adhesive that overcomes the disadvantages of the related art and demonstrates clearly superior properties.
This object is achieved, according to the present invention, by providing a radiationxe2x80x94and/or heat-curable adhesive with high thermal conductivity that is characterized by an adhesive formula that includes 19 to 40 wt % of a difunctional acrylated, aromatic polyurethane; 9 to 25 wt % of (2-hydroxypropyl) acrylate or methacrylate; 1 to 5 wt % of one or more photoinitiators, 35 to 70 wt % of one or more mineral fillers; 0.5 to 3 wt % of an SiO2-containing thixotropizing agent; and 0.5 to 2 wt % of an organic peroxide.
In one preferred embodiment, the photoinitiator is 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-ethyl-1-propane-1-one, bis(2,4,6-trimethyl benzoyl)phenyl phosphine oxide or 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propanone-1.
In a further preferred embodiment, the mineral filler is Al2O3, AlN, BeO, BN, SiC, Si3N4, TiO2, SiO2, diamond, graphite, BaSO4, GaAs or a mixture thereof. The mineral fillers used are thermally, yet not electrically, conductive mineral fillers.
In a further preferred embodiment, tert. butyl peroxy benzoate, methyl isobutyl ketone peroxide or dibenzoyl peroxide is used as the peroxide.
In a particularly preferred embodiment, the formula of the adhesive according to the present invention includes 21 to 28 wt % of a difunctional acrylated, aromatic polyurethane; 11 to 18 wt % of (2-hydroxypropyl) acrylate or methacrylate; 1 to 4 wt % of one or more photoinitiators, 50 to 65 wt % of one or more mineral fillers; 1 to 2.5 wt % of an SiO2-containing thixotropizing agent; and 1 to 1.5 wt % of an organic peroxide.
A radiationxe2x80x94and/or heat-curable adhesive having high thermal conductivity, characterized in that its formula contains the following components, is particularly preferred:
23 to 26 wt % of a difunctional acrylated, aromatic polyurethane; 13 to 16 wt % of (2-hydroxypropyl) acrylate or methacrylate; 2 to 3 wt % of one or more photoinitiators, 52 to 60 wt % of one or more mineral fillers; 1 to 2 wt. % of an SiO2-containing thixotropizing agent; and 1 to 1.5 wt % of an organic peroxide.
Actilane(copyright) 170, from Akcros Chemicals, can be used, for example, as the difunctional acrylated, aromatic polyurethane. The polyurethane used in this case has a relatively low molecular weight.
Known SiO2-containing thixotropizing agents used according to the present invention include, for example, the known Aerosils(copyright) by Degussa.
The single-component adhesive according to the present invention makes it possible to bond electronic components or circuits to heat sinks in a particularly advantageous manner. Examples of electronic components are transistors, or integrated circuits. Examples of heat sinks are metal elements.
The particular advantage of the adhesive according to the present invention lies in a combination of multiple advantageous properties. Thus, it is suitable for a long-term service temperature of up to 180 degrees Celsius. In addition, its thermal conductivity is 0.7 to 1.2 W/mK. Its adhesive strength is greater than 10 N/mm2 according to DIN EN 1465. Adhesive shrinkage is less than 6 wt% during radiation curing.
A particularly advantageous feature is that silicone gels with platinum catalysts are not inhibited when they come into contact with the adhesive. Furthermore, oxygen inhibiting does not occur on the adhesive surface during radiation and/or heat curing, which produces a dry surface. Unlike known adhesives according to the related art, it can be assumed that the monomers used are fully converted.
The adhesive according to the present invention is also characterized in that its applications are particularly flexible, since it is radiation-curable, heat-curable, and both radiationxe2x80x94and heat-curable. The novel adhesive is further characterized by particularly favorable processing characteristics. Thus, it can be punched and easily applied by screen printing. Furthermore, it is easy to dispense, i.e., it can be easily delivered, for example, by a needle jet.
In a further preferred embodiment, the adhesive is characterized in that it includes additional spherical spacers. The latter can be made of metal, plastic, glass, ceramic or a combination of these materials. Their diameter lies between 10 and 100 xcexcm, preferably between 20 and 40 xcexcm. Providing these spacers in the adhesive makes it possible to easily achieve a defined distance between the adhering parts.
The mineral filler particle size is less than 100 xcexcm. It is preferably between 1 and 20 xcexcm. The mineral filler particle size can determine the adhesive layer thickness.